1. Technical Field of the Invention
The present invention relates to bookbinding apparatuses, bookbinding systems, and adhesive coating methods that form booklets by coating an adhesive on the edge (spine) of a plurality of sheets that have been stacked, and, in particular, to bookbinding apparatuses, bookbinding systems, and adhesive coating methods that carry out adhesive coating operation for each individual booklet.
2. Description of Related Art
Bookbinding apparatuses that coat the adhesive for each booklet and bind the sheets are used frequently in bookbinding systems that carry out in one sequence the operations from image formation up to bookbinding. In such bookbinding apparatuses, hot melt type adhesives are used, and the binding operation is carried out continuously by replenishing small quantities of the pellets of the adhesive according to the consumption due to coating. In Patent Documents 1 and Patent Document 2, a replenishing apparatus has been disclosed in which the adhesive is coated on the stack of sheets by moving the coating roller along the spine of the stack of sheets, and pellets of adhesive are replenished at a constant rate in units of a few pellets to the molten adhesive reservoir in which the coating roller is dipped.
Patent Document 1: Unexamined Japanese Patent Application Publication No. 2004-209746.
Patent Document 2: Unexamined Japanese Patent Application Publication No. 2004-276457.
In a coating apparatus that coats the adhesive by moving the coating apparatus constituted to have a coating roller and an adhesive reservoir along the spine of the stack of sheets, since there is a restriction on the volume of the coating apparatus, even the volume of the adhesive reservoir will be restricted.
Because of this, the extent of lowering of the liquid level in the adhesive reservoir becomes large due to the reduction in the quantity of liquid that is consumed by coating, and in order to make the coating uniform, a means is required to suppress the up and down displacement of the liquid level.
In Patent Document 1 and Patent Document 2, a constant quantity of pellets is being supplied when the quantity of adhesive in the adhesive reservoir becomes small, and because of this, the adhesive liquid level is maintained constant. However, according to the adhesive replenishment method of Patent Document 1 and Patent Document 2, there will be up and down displacement of the adhesive liquid level, and it is difficult to carry out uniform coating. This is because of the following reasons.
This is explained below using FIGS. 1(a) to 1(c).
The liquid level LS of the molten adhesive is detected using an adhesive quantity sensor TS that detects the temperature, and, as is shown in FIG. 1(a) to FIG. 1(b) or FIG. 1(c), when the adhesive quantity sensor TS gets separated from the liquid level LS due to lowering of the liquid level LS, the temperature detected by the adhesive quantity sensor TS falls, and hence a lowering of the liquid level LS is detected.
When the volume of the adhesive reservoir BT is small, since the ratio of the quantity of adhesive scooped up by the coating roller RL to the quantity of adhesive in the adhesive reservoir is high, the fluctuations in the position of the liquid level is large, and when the thicknesses of the stack of sheets to be coated are different, there will be cases in which the state changes from that of FIG. 1(a) to FIG. 1(b) and from that of FIG. 1(a) to FIG. 1(c) due to one coating process.
Further, as is shown in the FIG., when the coating roller rotates as shown by the arrow mark, although the liquid level LS of the adhesive becomes high on the downstream side of the direction of rotation of the coating roller RL due to the viscosity of the liquid, the level of the liquid level LS on the downstream side is rather stable, and by detecting on the downstream side, it is possible to detect he quantity of adhesive in the adhesive reservoir accurately.
The adhesive quantity sensor TS cannot distinguish between the conditions of FIG. 1(b) and FIG. 1(c) by merely detecting that the state has changed from FIG. 1(a) to FIG. 1(b) and from FIG. 1(a) to FIG. 1(c). In other words, the adhesive quantity sensor TS carries out only ON/OFF detection, and cannot detect the amount of reduction in the level of the liquid level.
Therefore, when replenishment of adhesive is carried out using the detection signal from an adhesive quantity sensor such as in FIG. 1(a) to FIG. 1(c), due to the difference in the rate of consumption of the adhesive, the liquid level of the adhesive moves up and down, and it becomes difficult to carry out uniform coating.
If a plurality of adhesive quantity sensors is used, although it is possible to detect multiple levels of the liquid level, there are problems in terms of cost and the space for installing the adhesive quantity sensors, and the problem becomes more pronounced when increasing the accuracy of detection.
The present invention is intended to solve this kind of problems in replenishing adhesive to the coating apparatus, and the purpose of the present invention is to provide a bookbinding apparatus that makes it possible to coat uniformly using a coating apparatus that has been made small in size.